The spectral sensitivity of cat retinal ganglion neurons (RGNs) was determined by means of extracellular recordings under scotopic and photopic conditions, in both receptive field center and surround. Test stimuli were presented either as square-wave single flashes or as flicker stimuli. Chromatic adaptation was achieved by a large steady monochromatic background field. In the dark-adapted state the spectral sensitivity of the majority of ganglion cells (92%) was rod mediated (peak sensitivity at 501 nm). Under photopic conditions all neurons received input from a long-wavelength-sensitive (L-cone) system with a peak sensitivity of 550 nm. Input from a short-wavelength-sensitive (S-cone) system (peak sensitivity at 450 nm), however, was found only in 15% of the ganglion cells. A small cell population (8%) located within the area centralis revealed a different receptive field organization. In these cells, spectral sensitivity in the field center peaked at 520 nm in the dark-adapted state and response threshold was about 1 log unit higher than in cells with a peak sensitivity of 501 nm. Critical flicker fusion was reached at 60-70 Hz, a frequency that usually is mediated by cones. We therefore postulate an additional input of a midspectral receptor system (M-system) other than rods in cat retinal ganglion cells. This input was found only in the receptive field center of some ganglion cells in the dark-adapted state, whereas the surround sensitivity was mediated in all cells by rod signals under scotopic and predominantly by L-cone signals under photopic conditions.